Composite metal article having an intermediate bonding layer of nickel aluminide



United States Patent Int. Cl. B32b 15/00 US. Cl. 29-197 1 Claim ABSTRACTOF THE DISCLOSURE In a jet engine assembly, an abradable sealing surfacebetween the tips of rotating elements and the surrounding assembly,featuring the use of a bonding medium and a porous nickel abradablecoating wherein a graded sealing surface is achieved.

This is a division of the Emanuelson et a1. application, Ser. No. 438,734, filed Mar. 10, 1965.

This invention relates in general to the tip-sealing of rotatingelements in gas turbine engines and more particularly to an improvedabradable gas seal construction for such engines. It contemplates aprocess whereby a structurally reliable abradable coating is applied toengine stator assemblies by thermal spraying techniques.

The efficiency of a gas turbine engine is dependent to some extent uponthe control of gas leakage between stages in both the compressor andturbine sections of the engine. Although the engine is typicallydesigned and manufactured to very precise dimensional tolerances, it isnecessary to provide a sufiicient cold clearance between the tips of therotating elements and the surrounding stator assembly to accommodate thedifferential thermal growth between the parts as the engine assumes isnormal operating temperature. To this cold clearance must be added theusual manufacturing tolerances plus an additional safety factor toprovide for limited engine operation at temperatures in excess of thedesign temperature. The requisite clearances thus provided are, however,generally not sufiicienitly close to permit the engine to operate at itsmaximum theoretical efficiency.

In an effort to remedy this condition, it has been proposed to utilizean abradable surface on the assembly surrounding the rotating elementsand to permit the knifeedge of the rotor system to penetrate into thecoating as a result of thermal expansion, thereby permitting the rotorto seat itself against the stator assembly with what is essentially azero clearance. The result is considerably better sealing andconsequently better efliciency than that provided by conventional airseals. A typical abradable seal construction of this type is shown inthe patent to Koehring 2,930,521.

While in theory such an abradable surface may be seen to have greatpotential in improving engine performance, current techniques andabradable seal structures have not been entirely satisfactory in theirpractical application to high performance jet engines. In general, theproblems that have arisen in the use of abradable coatings areattributable to the lack of structural strength in the coating itselfand in its poor adherence to the metal substrate to which it is applied.

It is, therefore, an object of this invention to provide an abradablecoating which is characterized by a high strength bond with the metalsubstrate and good structural integrity in the elevated temperatureenvironment of a high performance jet engine. This objective is achievedby providing a coating in which the composition varies as a function ofits depth.

It is a further object of this invention to provide a procedure foreffecting such a coating utilizing thermal spraying techniques.

It is a still further object to provide a technique for varying thecharacteristics of the coating as a function of its depth .withoutintroducing shear planes therein. This technique contemplates varyingthe compositions of coating powders introduced to a spray gun as thecoating is effected on the metal substrate, or varying the respectivespray rates of a plurality of spray guns, to form an intermetall'ic atthe substrate surface and an abradable composition at the outer surfaceof the coating, gradually phasing from one composition to another as thecoating is produced.

These and other objects and advantages of the present invention will beevident or will be discussed in connection the following description.

It is axiomatic that the abradable surface of the coating must beselected in accordance with the anticipated environment in which it isto be run. A suitable surface material must be readily penetrated by therotating blade with a minimum of blade wear, and yet must Withstand, injet engine applications, the erosive and corrosive effects of the hotengine gases.

Abrasion of a material by a high speed rotating knifeedge occurs eitheras a result of the material being pushed aside or as a result ofparticles being broken away. Soft materials are abraded by the firstmethod and friable materials by the second method. It should be noted,however, that friable materials generally have reduced strengthproperties and are therefore not suitable for abradable coatings unlesssupported by a matrix of stronger materials.

The abradability of materials may be increased by decreasing thematerial density and increasing the porosity, although each of thesefactors reduces the strength of the coating and the resistance of thematerial to erosion. Further, the abradability may be increased byannealing. The hardness of nickel, for example, is reduced from aVickers hardness of over 200 to less than by annealing at 900 F.

As has been indicated, the abradability may be increased by introducingporosity into the coating, and this has been demonstrated to be thepreferable method. The abradability index increases with the volumepercent of pores, provided the pore size remains constant, but excessiveporosity will reduce the mechanical and bond strengths below thedesirable levels. Several methods have been utilized to introduceporosity to the material, one such method being the addition of a saltto the coating material as applied which may subsequently be leachedfrom the formed coating. An alternate method of effecting porosity is tointroduce a material into the coating as applied which may subsequentlybe removed by an oxidation process. A third method contemplates theaddition of a constituent to the coating mixture, such as boron nitride,which will form a brittle compound at the interparticle boundaries.

Although several methods of applying the coatings are available, themost satisfactory results have been obtained by means of thermalspraying. This process is inherently flexible since it permits thecoating material to be selected and premixed in powder form beforespraying, and because close control can be exercised over, the sprayingprocess.

Flame spraying is a coating process in which powdered material isinjected into a flame, melted, and subsequently carried in the moltenstate and deposited on a substrate by a gas jet. The flame is producedby the combustion of hydrogen or acetylene with oxygen or air. Thecarrier gas is usually air or one of the inert gases. The efficiency andquality of the coating processes depends on the flame temperature andheat production rate, the gas flow rates, the powder feed rate, and thedistance between the substrate and the spray gun. Adjustment of the gasflow rates changes the combustion rate and the time that the powderparticles remain in the combustion zone. Changing the substrate-to-gundistance changes the particle cooling time before impact and is probablythe most important spraying parameter.

Plasma spraying is similar to flame spraying except that the heat isderived from an electrical arc and no combustion products are produced.Considerably more energy is generated during plasma spraying than duringflame spraying, however, and the temperature produced is 40,000 F.whereas the maximum temperature produced by flame spraying is about 3600F. As a result, the coatings produced by the two processes have somewhatdifferent properties. For example, salt added to metallic powders toproduce porosity is vaporized during plasma spraying and consequentlyits usefulness as a porosity developer is lost.

Flame spraying has been demonstrated to be somewhat more suitable thanplasma spraying in the application of abradable coatings since lessenergy is added during the process, and the resulting coating is softerand less dense. Although the flame spraying process has been shown to bethe most suitable method of applying abradable coatings, it hasnevertheless been necessary to develop special spraying techniques andcoating compositions before truly suitable coatings are obtained. Earlyexperiments revealed that an adequate bond could not be formed betweenthe abradable coatings, for example, porous nickel, and the metalsubstrate. Mechanical methods, such as knurling the substrate, have beenattempted but were found to be impractical. In flame spraying the bondstrength is weakened by thermal stresses which develop at thecoating-substrate interface and, although the coefficients of thermalexpansion may be similar, the stresses are developed during theformation of the bond while the coating material and the substrate areat different temperatures. Consequently, it was found necessary tointerpose between the substrate and the abradable material anintermediate medium to improve the bonding to the substrate. It wasfurther found necessary to effect a gradual phasing of one componentinto the other as a function of the coating thickness in order toeliminate shear planes therein which lead to laminar separation in thecoating.

For turbine applications, nickel and chromium were selected as the mostsuitable abradable materials since they were known to have meltingpoints in excess of 2000 F. and were further known to be compatible withthe corrosive engine gases. Ceramics were not seriously considered sincethey are inherently brittle and not easily bonded to the non-rotatingengine parts. Alloys are generally unsuitable because of their tendencyto form low melting point eutectics or hard metallurgical phases whichreduces the abradability. Mixtures of nickel and silver, however, havebeen determined to be suitable candidate materials since no alloy orsolid solution is formed therebetween. The metals palladium, platinum,and rhodium are known to possess the requisite temperature and corrosionresistance characteristics but are not suitable for engine applicationsbecause of their cost.

The intermediate medium selected to form the bridge between substrateand the abradable surface is nickel aluminide which will bondmetallurgically to the substrate if properly applied. The nickelaluminide phase of the coating is formed on the substrate by means of aplasma spray utilizing nickel-coated aluminum powders, knowncommercially as METCO 404, as the feed material, the powder analyzing atapproximately 80% nickel and aluminum. Since the nickel and aluminummixture in this 4 form is both exothermic and synergistic, it will forma strong metallurgical bond with the metal substrate when applied from aplasma gun.

While the deposited nickel aluminide forms an excellent base for asubsequent topcoat, it was found that laminar separation could occur atthe nickel aluminideporous nickel interface during engine operation.Consequently a technique was developed whereby the coating was gradedgradually from pure nickel aluminide at the substrate surface to purenickel at the outer surface of the coating. This technique involvesspraying each constituent separately, the nickel coated aluminum powderfrom a plasma gun and the nickel from a flame gun, but sequentially toproduce the desired material gradient in the coating. Attempts to applya graded coating with a single gun by changing the characteristics ofthe powders in the charge was unsatisfactory because the constituentsrequired different spraying parameters. With different materials,however, particularly those having similar characteristics, there is noreason why a single gun could not be used for this purpose.

This process is suitable for effecting a coating on a variety of metalsubstrates, the only requirement being the establishment of ametallurgical bond between the substrate and the intermediate medium.For example, the nickel coated aluminum powders applied by plasma spraytechniques are known to bond to nickel base alloys, steels, aluminum,titanium, tantalum and columbium.

A porous nickel abradable surface has been applied to engine partsutilizing the above-mentioned technique by gradually phasing the coatingfrom nickel aluminide to a nickel salt mixture and subsequently leachingout the salt after formation of the coating.

The highest abradability was exhibited in tests on a coating with aporous surface containing 33% nickel but the excessive porosity resultedin a significant reduction in structural integrity. The optimum nickelconcentration in the abradable surface has been established at between50 and volume percent nickel.

EXAMPLE Nickel salt mixture:

Nickel powder% by weight (68.6% by volume) Salt (sodium chloride)-10% byweight (31.4% by volume) Aluminum phosphate1 cc. per 8 grams salt Thepowders fed to the flame spraying apparatus were prepared in theindicated proportions and were thoroughly blended,

Although abradability testing of coatings indicated that large grainsizes produced the more abradable coatings, the finer powders were shownto be more suitable for spraying. It is also advisable to abradeparticles from a coating which are fine enough not to damage downstreamengine parts. Accordingly, the powders utilized were in the range of200-250 mesh.

The nickel powder was previously reduced in a hydrogen-rich atmosphereto control the oxygen content since powders with an oxygen content of 3percent or more do not spray effectively.

Aluminum phosphate was added to the mixture to bind the salt crystals tothe metallic powder. Without the binder, the salt separates from themetallic powder in the vibrating hoppers used in conjunction with thespraying equipment.

Nickel aluminide powder:

METCO 404 Spray equipment Giannini plasma spray gun SG1:

Current-550 amperes Arc gas flow (argon)SS c.f.h. Powder gas (argon)27c.f.h.

Colmonoy spray gun: Oxygen-27 c.f.h.

Hydrogen58 c.f.h. Powder gas (nitrogen)-45 c.f.h.

Coating process After degreasing, Inconel engine parts to be coated weregrit blasted to roughen the surface. Silicon carbide #24 grit was usedfor this purpose. The surface to be coated was rotated at approximately47 inches per minute at a distance of approximately four inches from theplasma gun and preheated to 200 F. using the plasma torch with thepowder feed turned off. The nickel aluminide was applied to the rotatingpart to a thickness of .003.005 inch and, while the plasma gunapplication of nickel aluminide was continued, the nickel-salt mixturewas simultaneously phased in as a separate stream from the flame gun,the nickel aluminide feed rate being gradually decreased while thenickel-salt feed rate was increased. The transition from nickelaluminide to nickelsalt was effected within .015.020 inch of the Inconelsubstrate. The nickel-salt application was then continued until thedesired abradable surface thickness was attained. Although this is afunction of the design requirements, the abradable surface was generally.025 inch thick after machining.

The salt inclusions in the coating were then leached out with agitatedwater at 180 F. For engine applications it is essential that all of theexposed salt is removed from the finished coating since residualchlorine catalyzes a sulfidation reaction to which high temperature,high nickel content aircraft alloys are particularly vulnerable. Forthis reason, the salt removal is preferably undertaken before anymachining of the coating has been performed since metal flow duringmachining tends to entrap salt particles.

After the salt has been removed, any necessary machining was performedfollowed by annealing at 1300" F. to eliminate the surface workhardening produced in the machining operation.

Over 150 engine parts were coated with abradable 6 materials and testedin experimental engines. An abradable porous nickel coating of 30 volumepercent porosity bonded with nickel aluminide by the foregoing methodwas tested in the turbine section of an experimental engine anddemonstrated a capability of over 200 hours of engine service. Analysisindicates that an improvement in turbine efliciency of at least 1 /2percent is realized.

While a preferred process and abradable seal composition has beendescribed, the present invention in its broader aspects is not limitedthereto but departures may be made from such details within the scope ofthe accompanying claim.

We claim:

1. A composite comprising a metallized aircraft engine componentfabricated from a high temperature nickelbase alloy having an abradablesurface coating thereon, the composite comprising a base formed of anickelbase alloy, an intermediate bonding zone of nickel aluminidemetallurgically bonded to said base, and an outer layer of porous nickelof 2050 volume percent porosity, said outer layer being bonded to thenickel aluminide intermetallic layer by a bond zone that increases inporous nickel as it recedes from the intermediate layer.

References Cited UNITED STATES PATENTS 2,763,920 9/1956 Turner 29-1983,041,040 6/1962 Levinstein 29--l98 3,129,069 4/1964 Hanink 29----l943,141,744 7/1964 Couch 29--197 3,337,427 8/1967 Whitfield 29--1982,996,795 8/1961 Stout 29--191.2 3,322,515 5/1967 Dittrich 29-192 HYLANDBIZOT, Primary Examiner US. Cl. X.R. 29194

